Long-term Stability of Circulating Nucleosomes in Serum

Background: Circulating nucleosomes, complexes of DNA and histones, are recognized as a potential new diagnostic tool for therapy monitoring of cancer disease and as prognostic marker in other acute diseases. Because many studies are carried out retrospectively on serum samples collected earlier, we tested the stability of nucleosomes during long-term storage at -70 degrees C. Patients and Methods: Never-thawed aliquots of 154 stabilized serum samples from cancer patients which had been stored between 11/1999 and 07/2001 at -70 degrees C were analyzed by ELISA six to nine months after collection and for a second time in 11/2006, with a median interval of 64.8 months (+/-5.5 months) between the measurements. Results: Nucleosome levels of the second measurement (median 220.0 nglml) were significantly lower than those of the first measurement (354.0 ng/ml; p<0.0001) exhibiting a median decrease of 32.0% (mean 29.7%+/-39.7%). Single concentrations of both measurements correlated well (R=0.92; p<0.0001). Individual courses of eight cancer patients showed a parallel downshifting of the nucleosome signal. The concordance rate of parallel increases and decreases was 89%. Conclusion: Retrospective studies on nucleosomes in serum can be performed if samples derive from homogeneous dine intervals and preanalytical protocols are respected.